Initiation of the myogenic program in adult muscle is a critical step in skeletal muscle regeneration, which is essential for muscle maintenance and adaptation. Muscle regeneration requires chromatin remodeling in myogenic cells that allows transcriptional activation of myogenic target genes. The activation of p38 MAPK plays a critical role in chromatin remodeling and in the activation of key myogenic transcription factors and, thus, is considered a molecular switch for the activation of myogenesis. However, the signaling mechanism that mediates p38 activation in response to diverse myogenic stimuli in adult muscle remains poorly understood. Supported by R01 AR049022, we have discovered that myogenic activation of p38 is mediated by TNF-1 converting enzyme (TACE) through the release of TNF-1, a cytokine that activates p38, from myogenic cells as an autocrine factor. These discoveries reveal a new paradigm of the regulation of myogenesis. Nevertheless, to fully establish this new paradigm, the mechanism of myogenic activation of TACE must be delineated. In preliminary studies, we obtained initial evidence that two mechanisms mediate myogenic activation of TACE. One involves a down-regulation of tissue inhibitor of metallopreoteinase3 (TIMP3), which removes the suppression on TACE. The other involves tyrosine phosphorylation of TACE that augments TACE activity. Based on our preliminary data, the current project will evaluate the signaling mechanisms that mediate myogenic activation of TACE by pursuing three specific aims. Aim 1. To determine whether TIMP3 regulates myogenesis via its inhibition of TACE. We will test the conceptual model in cultured myoblasts and adult muscle that as an endogenous inhibitor of TACE, constitutively expressed TIMP3 suppresses myogenic differentiation; when stimulated by myogenic stimuli, TIMP3 undergoes a down-regulation which removes the inhibition on TACE and permits myogenesis to take place. We will also elucidate the mechanisms through which TIMP3 is down-regulated. Aim 2. To determine whether Src mediates TACE activation by myogenic stimuli. We will test the conceptual model that TACE is activated via phosphorylation by the tyrosine protein kinase Src when stimulated by myogenic stimuli; and this reaction is critical to myogenesis. Aim 3. To determine whether 23 integrin mediates myogenic activation of Src. We will test the conceptual model that the 23 integrin serves as the membrane receptor that transduces mechanochemical cues of myogenesis into myogenic cells through recruiting and activating Src. Thus, a 23 integrin Src TACE pathway that mediates the myogenic activation of p38 can be established. Our long term goal is to identify means for promoting muscle regeneration via the understanding of the signaling mechanisms that regulate muscle regeneration. PUBLIC HEALTH RELEVANCE. Muscle regeneration is essential for muscle maintenance and adaptation. The goal of the current grant application is to identify means for promoting muscle regeneration via the understanding of the signaling mechanisms that regulate myogenesis by activating p38 MAPK in adult muscle.